Synthesis of enantiomerically enriched β-hydroxy selenides by catalytic asymmetric ring opening of meso-epoxides with (phenylseleno)silanes

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Tetrahedron 64 (2008) 3337e3342 www.elsevier.com/locate/tet

Synthesis of enantiomerically enriched b-hydroxy selenides by catalytic asymmetric ring opening of meso-epoxides with (phenylseleno)silanes Marcello Tiecco*, Lorenzo Testaferri, Francesca Marini*, Silvia Sternativo, Francesca Del Verme, Claudio Santi, Luana Bagnoli, Andrea Temperini Dipartimento di Chimica e Tecnologia del Farmaco, Sezione di Chimica Organica, Universita` di Perugia, 06123 Perugia, Italy Received 6 November 2007; received in revised form 16 January 2008; accepted 31 January 2008 Available online 3 February 2008

Abstract The first example of the enantioselective ring opening of meso-epoxides by (phenylseleno)silanes using salen(Cr)complexes as catalyst is described. This desymmetrization reaction constitutes a simple and convenient approach to synthetically versatile optically active b-hydroxy selenides. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Desymmetrizations; Enantioselective synthesis; Organoselenium compounds; meso-Epoxides

1. Introduction b-Hydroxy selenides are useful intermediates for a variety of synthetic transformations. Together with classical reactions, such as the reductive deselenenylation to alcohols1 or the oxidation to allylic alcohols,1 simple and convenient conversions into tetrahydrofurans,2 1,3-oxazolidinones,3 amino alcohols,3 and b-aryl-selenides4,5 have been recently reported in the literature. In view of their large synthetic utilization, several approaches to enantiomerically enriched b-hydroxy selenides with different skeleton and stereochemical requirements have been developed. The most common procedures (Scheme 1) are the lipasepromoted kinetic resolutions of racemic b-hydroxy selenides (path a),6 the regio and stereospecific ring opening of commercial enantiopure epoxides by arylselenolates1 (substrate-controlled asymmetric syntheses, path b), the asymmetric seleno-hydroxylation of alkenes promoted by enantiomerically pure electrophilic

selenium reagents,7 and the asymmetric aldol reactions with enantiomerically pure a-seleno ketones, esters, or amides8 (reagentcontrolled asymmetric syntheses, path c and path d). O R3SeNa

R2 rac

SeR3

enzymatic kinetic resolution

R2=H

path a R2=H path f O R3SeH + 1 R

0040-4020/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2008.01.126

R2

path b

*

2 1 * R (R )

SeR3 chiral catalyst

R1

NaBH4 path e

N H

+

R1

R3SeX enantiopure

path c CH3CN, H2O

OH R1

R1=H O

* Corresponding authors. Tel.: þ39 075 585 5100; fax: þ39 075 585 5116 (M.T.); tel.: þ39 075 585 5105; fax: þ39 075 585 5116 (F.M.). E-mail addresses: [email protected] (M. Tiecco), [email protected] (F. Marini).

R2

R1

enantiopure

OH R1

+

path d TiCl4, Et3N R2

O R1

H

+

SeR3

enantiopure R2=COR, CO2R, CONRR'

+ R3SeX

R1 R2

Scheme 1. Preparation of optically active b-hydroxy selenides.

M. Tiecco et al. / Tetrahedron 64 (2008) 3337e3342

3338

Catalyst-controlled asymmetric syntheses have been scarcely investigated. Very recently, we have prepared b-hydroxy selenides in excellent enantiomeric purity by the chiral amine catalyzed a-selenenylation of aldehydes followed by in situ reduction (path e).9 b-Hydroxy selenides with two contiguous chiral centers have been prepared by the enantioselective ring opening of meso-epoxides with benzeneselenol in the presence of the heterobimetallic salen TitaniumeGallium complex (path f).10 We now report the first desymmetrization of meso-epoxides by (phenylseleno)silane 2a and commercial or easily accessible salen catalysts.11 This nucleophilic selenium source is more easy to handle than the easily oxidizable and malodorous benzeneselenol. 2. Results and discussion The (phenylseleno)silanes 2aec employed for the present investigation were prepared by analogy with the procedures described in the literature as indicated in Scheme 2.12 These compounds can be purified by distillation or crystallization and stored for several days without particular precautions. (PhSe)2 NaH THF PhSeNa

R1 Cl Si R2 R1

R1 PhSe Si R2 R1

2a R1= Me; R2 = t-Bu 2b R1= i-Pr; R2 = i-Pr 2c R1= Ph; R2 = t-Bu

Scheme 2. Preparation of the (phenylseleno)silanes 2aec.

Preliminary experiments were performed, at room temperature and under air atmosphere, on stilbene oxide 1a in order to evaluate the efficiency of the selenium reagents 2aec13 and of several (R,R)-salen(metal)complexes in different solvents (0.5 M solutions). The best results in terms of yield and er were obtained using the tert-butyl(dimethyl)(phenylseleno)silane 2a. Under the same reaction conditions 2b gave similar er, but considerably poorer yields, and 2c was ineffective. The results of several selected experiments carried out with 2a are reported in Table 1. The best results in terms of yield and enantiomeric ratio were obtained with salen(Cr)BF4 in tert-butylmethyl ether (entry 9). Further optimization of the reaction was then attempted. The effects of the temperature and the addition of 1 equiv of tert-butanol,1 tetrabutylammonium fluoride,14 or tetramethylethylendiamine (TMEDA),15 as activating agents for the silylated organoselenium reagents, have been investigated (entries 10e13). The results of these experiments indicate that the use of a low temperature and TMEDA as additive can dramatically improve the yield and the selectivity of the reaction (entry 11). In this case 3a was obtained with 92% yield and 96:4 er. These results are considerably better than those previously reported.10 Further improvements were not observed by using 10% of the catalyst, more concentrated solutions (2 M), or lower reaction temperatures. The stereochemical attribution of compound 3a was effected by reductive deselenenylation with tributyltin hydride and AIBN in refluxing toluene. The 1,2-diphenylethanol thus obtained has the R configuration as demonstrated

Table 1 Asymmetric ring opening of stilbene oxide 1a: optimization of the reaction conditions O Ph

Ph

+

1a

Entry Catalyst 1 2 3 4 5 6 7 8 9 10 11 12 13 a b

Salen(Ti)(OiPr)2 Salen(Zn) __ Salen(Cr)SbF6 Salen(Cr)Cl Salen(Cr)Cl (Salen)CrCl Salen(Cr)BF4 Salen(Cr)BF4 Salen(Cr)BF4 Salen(Cr)BF4 Salen(Cr)BF4 Salen(Cr)BF4 Salen(Cr)BF4

SePh HO Catalyst 5 mol% Si r.t. 96 h Ph 2a

SePh Ph 3a

Temperature Solvent (additive)

Yielda (%) erb

RT RT RT RT RT RT RT RT RT RT 10  C 10  C 10  C

46 69 73 69 60 60 60 67 82 99 92 99 35

Hexane Hexane TBME Hexane Toluene TBME Hexane Toluene TBME TBME (TMEDA) TBME (TMEDA) TBME (t-BuOH) TBME (Bu4NF)

64:36 50:50 69:31 82:18 77:23 63:37 84:16 89:11 89:11 73:27 96:4 77:23 87:13

Yields determined on isolated compounds after column chromatography. Enantiomeric ratios determined by HPLC on a Chiralpack AD-H column.

by the comparison of the specific optical rotation with that reported in the literature.16 Considering that 3a is formed with complete anti diastereoselectivity it can be confidently assumed that 3a has the absolute configuration as indicated in Table 1. We next examined the scope of this ring opening reaction by carrying out the desymmetrization reactions on other meso-epoxides. The epoxides 1feh are commercially available. The aryl epoxides 1bee were prepared17 from substituted benzyl iodides and appropriate commercial benzaldehydes via Wittig reactions. The crude alkenes were submitted to epoxidation with m-CPBA without purification. cis-Epoxides, which under the reaction conditions employed are the major or the sole reaction products, were obtained in pure form after column chromatography. The results of the desymmetrization experiments of the epoxides 1beh are reported in Table 2. The results obtained on stilbene oxide 1a under optimized reaction conditions (method A) are also reported for comparison. The absolute configurations of compounds 3bee were assigned by analogy with 3a, whereas those of compounds 3feh were attributed by comparison of the HPLC retention times (Chiralcel OD-H column) with those previously reported in the literature.10 In order to obtain good levels of enantioselectivity it was essential to use low reaction temperatures. Notably the enantioselectivity of the process depends on the structure of the starting epoxide. In fact, higher enantiomeric ratios were obtained with stilbene oxide 1a and the p-substituted aryl epoxides 1bed. As already observed for 1a the addition of TMEDA (method A) significantly improved the efficiency of the process in the cases of the aryl epoxides 1bee (entries 2e5). Cyclic or alkyl epoxides gave instead moderate to good results when TMEDA was not added (method B).

M. Tiecco et al. / Tetrahedron 64 (2008) 3337e3342

3339

Table 2 Asymmetric ring opening of meso-epoxides 1aeh Entry

Epoxide

Product

O

1

SePh

HO Ph

Ph

Ph

1a O

Ph

3a

HO

F

1b

3b

F

O

HO

Br

1c

HO

CN

1d

Seph

HO

O

SePh

1f

SePh

HO

7 1g

A 53

86:14

RT

A 31

10  C

A 25

a

72:28

b b

84:16

RT RT

A 70 a,b A 53

80:20 81:19

10  C RT RT

A 50 A 94a A 34

75:25 66:34 64:36

10  C 10  C

A 52 B 71

64:36 75:25

10  C 10  C

A 70 B 88

67:33 81:19

10  C

B 60

76:24

3g

O 1h

10  C

3f

O

H3C

85:15

3e

6

8

A 50

OMe MeO

HO

O

10  C

SePh

5 OMe MeO 1e

96:4

CN

3d

NC

A 92

Br

4 NC

10  C

SePh

3c

Br

O

er

F

3 Br

Yield (%)

SePh

2 F

Temperature

CH3

HO

SePh

H3C

CH3

3h

Method A: reactions carried out with salen(Cr)BF4 of 5 mol % in TBME in the presence of 1 equiv of TMEDA for 96 h. Method B: reactions carried out with salen(Cr)Cl of 5 mol % in Et2O, without TMEDA for 96 h. a Reaction carried out in the absence of TMEDA following method A. b Reaction carried out in CH2Cl2 following method A.

3. Conclusions

4. Experimental

In conclusion we have described the first enantioselective ring opening of meso-epoxides by (phenylseleno)silanes as nucleophilic selenium source and commercial or readily available salen(chromium) complexes as catalysts. These reactions constitute a simple and convenient approach to the synthetically versatile, optically active b-hydroxy selenides. The enantioselectivity of the process depends on the structure of the starting epoxide. Good to excellent results (up to 92% yield and 96:4 er) were obtained on stilbene epoxide and on its p-substituted derivatives in the presence of TMEDA as additive.18 The results described in this paper nicely complement those obtained using benzeneselenol as the nucleophile, which were particularly efficient in the desymmetrization of cyclic or alkyl epoxides.

4.1. General New compounds were characterized by 1H, 13C NMR, and mass spectra. 1H and 13C NMR spectra were recorded on a Bruker Avance-DRX 400 instrument (at 400 and 100.62 MHz, respectively) with CDCl3 as solvent and TMS as internal reference. GCeMS analyses were carried out with an HP 6890 gas chromatograph (HP-5MS capillary column 30 m, ID 0.25 mm, film 0.25 mm) equipped with an HP 5973 Mass Selective Detector; for the ions containing selenium only the peaks arising from the selenium-80 isotope are given. HPLC analyses were performed on an HP 1100 instrument equipped with a chiral column and an UV detector.

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M. Tiecco et al. / Tetrahedron 64 (2008) 3337e3342

Optical rotations were measured with a Jasco DIP-1000 digital polarimeter. Elemental analyses were carried out on a Carlo Erba 1106 Elemental Analyzer. Thin layer chromatography (TLC) was performed on silica gel 60 F254 (Merck) on aluminum sheets. Column chromatography was performed using silica gel Merck 60 (70e230 mesh). For flash chrotography silica gel Merck 60 (230e400 mesh) was used. 4.2. Synthesis of the meso-epoxides Epoxides 1a and 1feh are commercial compounds (Aldrich). Epoxides 1bee have been prepared from substituted benzyl iodides and appropriate commercial benzaldehydes via a Wittig reaction followed by oxidation with m-CPBA according to the procedure described in the literature.17 Intermediates have been employed without any purification. Chemical overall yields determined after purification by column chromatography, and physical and spectral data of compounds 1bed are reported below. GCeMS spectra of compounds 1b and 1c are not reported since these products suffered decomposition during the analysis. Compound 1e has spectral properties identical to those previously described.17 4.2.1. cis-2,3-Bis(4-fluorophenyl)oxirane (1b) This compound was obtained in pure form (45% yield, 0.95 g) after flash chromatography (light petroleum to light petroleum/ diethyl ether 96:4). Oil; Rf 0.60 (light petroleum/diethyl ether 80:20); 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.20e 7.00 (m, 4H, Ph), 7.0e6.75 (m, 4H, Ph), 4.25 (s, 2H, CHO); 13 C NMR (100 MHz, CDCl3, 25  C, TMS): d¼162.1 (d, 2C, 1 JCF¼244.3 Hz), 129.9 (2C), 128.4 (d, 4C, 3JCF¼8.0 Hz), 114.8 (d, 4C, 2JCF¼21.5 Hz), 59.0 (2C). Anal. Calcd for C14H10F2O: C, 72.41; H, 4.34. Found: C, 72.54; H, 4.19. 4.2.2. cis-2,3-Bis(4-bromophenyl)oxirane (1c) This compound was obtained in pure form (36% yield, 0.9 g) after column chromatography (light petroleum to light petroleum/diethyl ether 90:10). White solid; mp¼84e86  C; Rf 0.58 (light petroleum/diethyl ether 80:20); 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.30e7.20 (m, 4H, Ph), 7.02e6.80 (m, 4H, Ph), 4.25 (s, 2H, CHO); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼133.0 (2C), 131.2 (4C), 128.5 (4C), 121.8 (2C), 59.2 (2C). Anal. Calcd for C14H10Br2O: C, 47.50; H, 2.85. Found: C, 47.35; H, 2.78. 4.2.3. cis-2,3-Bis(4-cyanophenyl)oxirane (1d) This compound was obtained in pure form (30% yield, 0.7 g) after column chromatography (light petroleum/diethyl ether 70:30 to diethyl ether). White solid; mp¼172e174  C; Rf 0.15 (light petroleum/diethyl ether 60:40); 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.50e7.26 (m, 4H, Ph), 7.25e7.01 (m, 4H, Ph), 4.45 (s, 2H, CHO); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼138.4 (2C), 131.4 (4C), 126.9 (4C), 117.8 (2C), 111.5 (2C), 58.7 (2C); MS (70 eV, EI): m/z (%) 246 (100) [Mþ], 217 (74), 190 (35), 130 (33), 115 (63), 88 (22). Anal. Calcd for C16H10N2O: C, 78.03; H, 4.09; N, 11.38. Found: C, 77.87; H, 4.19; N, 11.50.

4.3. Synthesis of the (phenylseleno)silanes (Phenylseleno)silanes 2aec were prepared according to the literature procedure.12 Spectral data of compounds 2a and 2c are identical to those previously described. In the case of compound 2b the by-products were removed by distillation under vacuum and the residue was sufficiently pure to be directly employed for the ring opening reactions. Physical and spectral properties of compound 2b are reported below. 4.3.1. Triisopropyl(phenylseleno)silane 2b Yield 40%, 1.4 g; yellow oil, slightly impure; 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.70e7.65 (m, 2H, SePh), 7.30e7.14 (m, 3H, SePh), 1.36e1.28 (m, 3H, CH), 1.13 (d, 18H, J¼7.4 Hz, CH3); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼137.1 (2C), 128.6 (2C), 126.9, 124.7, 18.6 (6C), 13.2 (3C); MS (70 eV, EI): m/z (%) 314 (56) [Mþ], 271 (100), 243 (14), 229 (53), 215 (13), 201 (40), 187 (20), 157 (30), 123 (24), 105 (34), 77 (28), 59 (31). 4.4. Catalysts The (R,R)-salen(Cr)Cl is commercially available. The (R,R)-salen(Ti)(OiPr)2,11f the (R,R)-salen(Zn)(II),19 the (R,R)salen(Cr)BF4,20 and the (R,R)-salen(Cr)SbF621 have been prepared according to standard procedures. 4.5. Asymmetric ring opening of meso-epoxides: method A and method B Method A: the meso-epoxides 1aeg (0.5 mmol) and (R,R)-salen(Cr)BF4 (0.025 mmol) were dissolved in TBME (0.5 M), and then 2a (0.61 mmol) and TMEDA (0.5 mmol) were added. The resulting mixture was stirred for 96 h and then filtered under vacuum through a plug of silica gel with 30 mL of light petroleum/diethyl ether 70:30. The filtrate was concentrated and the crude mixture was analyzed by NMR. The crude b-hydroxy selenides 3aeg were purified by flash chromatography. Method B: the meso-epoxides 1feh were treated under the experimental conditions described for method A, but using salen(Cr)Cl as catalyst in Et2O and in the absence of TMEDA. Physical and spectral data of 3feh are comparable with those reported in the literature.10 Physical and spectral data of compounds 3aee are described below together with the eluant employed for the purification. 4.5.1. (1S,2S)-1,2-Diphenyl-2-(phenylseleno)ethanol (3a) This compound was obtained in pure form (92% yield, 165.6 mg) after flash chromatography (light petroleum to light petroleum/diethyl ether 90:10). Oil; Rf 0.42 (light petroleum/diethyl ether 80:20); [a]23 D þ152.9 (c 3.1, CHCl3); HPLC (Chiralpack AD-H (2504.6 mm ID), eluant: hexane/iPrOH 95:5, flow rate: 1 mL/min, UV detection at 230 nm), tR: minor enantiomer 24.3 min, major enantiomer 30.2 min. 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.48e7.44 (m, 2H, SePh), 7.30e7.20 (m, 11H, Ph, SePh), 7.03e6.99 (m, 2H, SePh), 5.08 (d, 1H,

M. Tiecco et al. / Tetrahedron 64 (2008) 3337e3342

J¼8.8 Hz, CHO), 4.53 (d, 1H, J¼8.8 Hz, CHSe), 3.40 (br s, 1H, OH); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼140.8, 139.9, 135.3 (2C), 128.9 (2C), 128.7, 128.4 (2C), 128.0 (4C), 127.9, 127.7, 127.0, 126.7 (2C), 76.6, 59.9. Anal. Calcd for C20H18OSe: C, 67.99; H, 5.14. Found: C, 68.21; H, 5.21. 4.5.2. (1S,2S)-1,2-Bis(4-fluorophenyl)-2-(phenylseleno)ethanol (3b) This compound was obtained in pure form (50% yield, 97.5 mg) after flash chromatography (light petroleum to light petroleum/diethyl ether 80:20). Oil; Rf 0.65 (light petroleum/ diethyl ether 60:40); [a]23 D þ110.5 (c 1.1, CHCl3); HPLC (Chiralpack AD-H (2504.6 mm ID), eluant: hexane/iPrOH 90:10, flow rate: 1 mL/min, UV detection at 230 nm), tR: minor enantiomer 11.98 min, major enantiomer 13.29 min. 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.35e7.25 (m, 2H, SePh), 7.25e7.0 (m, 5H, Ph, SePh), 6.90e6.70 (m, 6H, Ph), 4.92 (d, 1H, J¼8.6 Hz, CHO), 4.30 (d, 1H, J¼8.6 Hz, CHSe), 3.1 (br s, 1H, OH); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼162.2 (d, 1JCF¼244.8 Hz), 161.6 (d, 1 JCF¼245.0 Hz), 136.2 (d, 4JCF¼3 Hz), 135.5 (2C), 135.3 (d, 4 JCF¼3 Hz), 129.9 (d, 2C, 3JCF¼8.0 Hz), 129.0 (2C), 128.3 (d, 2C, 3JCF¼8.0 Hz), 128.2, 128.1, 115.0 (d, 4C, 2 JCF¼21.3 Hz), 75.9, 58.7; MS (70 eV, EI): m/z (%) 390 (<1), 266 (47), 233 (18), 216 (40), 201 (19), 185 (44), 158 (27), 137 (13), 123 (79) 109 (100), 95 (33), 77 (21), 51 (12). Anal. Calcd for C20H16F2OSe: C, 61.70; H, 4.14. Found: C, 61.61; H, 4.28. 4.5.3. (1S,2S)-1,2-Bis(4-bromophenyl)-2-(phenylseleno)ethanol (3c) This compound was obtained in pure form (53% yield, 135.7 mg) after flash chromatography (light petroleum to light petroleum/diethyl ether 90:10). Oil; Rf 0.44 (light petroleum/ diethyl ether 80:20); [a]23 D þ42.0 (c 1.1, CHCl3); HPLC (Chiralpack AD-H column (2504.6 mm ID), eluant: hexane/ i PrOH 90:10, flow rate: 0.75 mL/min, UV detection at 230 nm), tR: minor enantiomer 21.41 min, major enantiomer 23.76 min. 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.35e7.20 (m, 9H, Ph, SePh), 7.07e7.01 (m, 2H, Ph), 6.85e6.80 (m, 2H), 4.95 (d, 1H, J¼8.7 Hz, CHO), 4.36 (d, 1H, J¼8.7 Hz, CHSe), 3.30 (br s, 1H, OH); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼139.7, 138.6, 135.5 (2C), 131.2 (4C), 130.0 (2C), 129.0 (2C), 128.4 (2C), 128.3, 127.9, 121.7, 120.9, 75.5, 58.4; MS (70 eV, EI): m/z (%) 355 (24) [MþPhBr], 326 (86), 281 (36), 247 (40), 207 (69), 185 (100), 169 (59), 155 (41), 139 (16) 118 (18), 91 (25), 77 (39), 63 (16). Anal. Calcd for C20H16Br2OSe: C, 47.00; H, 3.16. Found: C, 47.12; H, 3.27. 4.5.4. (1S,2S)-1,2-Bis(4-cyanophenyl)-2-(phenylseleno)ethanol (3d) This compound was obtained in pure form (25% yield, 50.5 mg) after flash chromatography (CH2Cl2/MeOH 99:1 to CH2Cl2/MeOH 98:2). White solid; mp¼113e117  C; Rf 0.49 (CH2Cl2/MeOH 98:2); [a]23 D þ43.5 (c 1.0, CHCl3); HPLC (Chiralpack AD-H column (2504.6 mm ID), eluant:

3341

hexane/iPrOH 75:25, flow rate: 0.75 mL/min, UV detection at 230 nm), tR: minor enantiomer 17.53 min, major enantiomer 25.97 min. 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.52e7.48 (m, 2H, Ph), 7.45e7.40 (m, 2H, Ph), 7.35e7.19 (m, 7H, Ph, SePh), 7.08e6.95 (m, 2H), 5.07 (d, 1H, J¼8.3 Hz, CHO), 4.37 (d, 1H, J¼8.3 Hz, CHSe), 3.50 (br s, 1H, OH); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼145.6, 144.8, 135.9 (2C), 132.1 (4C), 129.3 (2C), 129.0 (2C), 128.9, 127.4 (2C), 127.0, 118.4 (2C), 112.0, 111.2, 75.2, 58.4. Anal. Calcd for C22H16N2OSe: C, 65.51; H, 4.00; N, 6.95. Found: C, 65.66; H, 4.17; N, 6.83. 4.5.5. (1S,2S)-1,2-Bis(3-methoxyphenyl)-2-(phenylseleno)ethanol (3e) This compound was obtained in pure form (50% yield, 103.5 mg) after flash chromatography (light petroleum to light petroleum/diethyl ether 70:30). Oil; Rf 0.44 (light petroleum/ diethyl ether 60:40); [a]23 D þ52.13 (c 1.1, CHCl3); HPLC (Chiralpack AD-H (2504.6 mm ID), eluent: hexane/iPrOH 90:10, flow rate: 1 mL/min, UV detection at 230 nm), tR: major enantiomer 34.86 min, minor enantiomer 37.60 min. 1H NMR (400 MHz, CDCl3, 25  C, TMS): d¼7.45e7.35 (m, 2H, SePh), 7.35e6.95 (m, 5H, Ph, SePh), 6.75e6.45 (m, 6H, Ph), 4.96 (dd, 1H, J¼2.5, 8.6 Hz, CHO), 4.38 (d, 1H, J¼8.6 Hz, CHSe), 3.64 (s, 3H, OMe), 3.60 (s, 3H, OMe), 2.66 (d, 1H, J¼2.5 Hz, OH); 13C NMR (100 MHz, CDCl3, 25  C, TMS): d¼159.3, 159.2, 142.4, 141.4, 135.3 (2C), 129.1, 129.0, 128.9 (2C), 128.7, 128.0, 120.9, 119.2, 113.9, 113.5, 112.8, 112.1, 76.3, 59.7, 55.1 (2C); MS (70 eV, EI): m/z (%) 278 (58), 240 (24), 227 (23), 207 (42), 195 (38), 165 (19), 149 (13), 135 (92), 121 (100) 107 (23), 91 (18), 77 (25). Anal. Calcd for C22H22O3Se: C, 63.92; H, 5.36. Found: C, 63.79; H, 5.18. Acknowledgements Financial support from MIUR, National Projects PRIN2003, PRIN2005 and FIRB2001, and Consorzio CINMPIS is gratefully acknowledged. References and notes 1. (a) Topics in Current Chemistry: Organoselenium Chemistry: Modern Developments in Organic Synthesis; Wirth, T., Ed.; Springer: Heidelberg, 2000; (b) Organoselenium ChemistrydA Practical Approach; Back, T. G., Ed.; Oxford University Press: New York, NY, 2000. 2. (a) Ericsson, C.; Engman, L. Org. Lett. 2001, 3, 3459; (b) Gupta, V.; Engman, L. J. Org. Chem. 1997, 62, 157; (c) Tiecco, M.; Testaferri, L.; Bagnoli, L.; Purgatorio, V.; Temperini, A.; Marini, F.; Santi, C. Tetrahedron: Asymmetry 2004, 15, 405; (d) Tiecco, M.; Testaferri, L.; Marini, F.; Sternativo, S.; Santi, C.; Bagnoli, L.; Temperini, A. Tetrahedron 2007, 63, 5482. 3. Tiecco, M.; Testaferri, L.; Temperini, A.; Bagnoli, L.; Marini, F.; Santi, C. Chem.dEur. J. 2004, 10, 1752. 4. Okamoto, K.; Nishibayashi, Y.; Uemura, S.; Toshimitsu, A. Tetrahedron Lett. 2004, 45, 6137. 5. Toshimitsu, A.; Nakano, K.; Mukai, T.; Tamao, K. J. Am. Chem. Soc. 1996, 118, 2756. 6. (a) Costa, C. E.; Clososki, G. C.; Barchesi, H. B.; Zanotto, S. P.; Nascimento, M. G.; Comasseto, J. V. Tetrahedron: Asymmetry 2004, 15, 3945;

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7.

8.

9. 10. 11.

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